Blackouts and brownouts of the electric utility systems have been occurring with greater frequency because of the energy crisis. Storms also cause power failures which in this age of dependence upon public power systems leaves many essential functions vulnerable. Amongst these essential functions are the filling of reservoirs and other supply tanks for water systems, lift pumps for sewage systems, oil pumping stations, compressors for refrigeration plants, and fuel pumps for large heating systems.
Conventional stand-by power units are designed to supply electrical power to the portion of the load to which they are assigned in the same manner as the electrical utility systems. A smooth transition of transfer of the electrical load from the supply main to the stand-by power system and vice versa allowing time for starting and stopping the stand-by engine has been a matter of design refinements throughout many years. In such a system, the stand-by engine drives an electrical generator the output of which must be connected and disconnected from the load by switch gear. Such switches must not only be relatively free from damage due to arcing, in breaking contacts under electrical load, but also due to welding of contacts together by the load. In some systems, switches open as the voltage in the main drops to 33% of normal and close when this voltage returns to 80% of normal voltage. If the load includes incandescent lamps, the inrush current for the lamps can be 10 to 20 times the normal full-load rating and the power transfer switches as well as the generator and stand-by engine must be designed and rated for these high inrush loads. Selection of components and overall design of the conventional stand-by system thus has relied heavily upon electrical generators, controls, switches, engine starters, storage batteries which must be replaced every few years, and battery charging systems even where the load need not be electrical. These components require highly paid; skilled technicians for maintaining, repairing, and frequent reliability checking. Such workers are few and being specialists are not always readily available in an emergency. The reliability of this type of system is thus dependent upon the availability of sophisticated components and the services of skilled technicians. The capital and operating cost of this equipment can be high because of design refinements and high inrush load capacity in comparison to normal operating load.